Interbuilding and urban canyon extension solution for global positioning systems

ABSTRACT

A global positioning system (GPS) and auxiliary receiver receive limited range position signals broadcasted from telemetry transmitters when in an urban canyon environment. The limited range receivers preferably are implemented as part of the lighting infrastructure of the environment. The limited range receivers derive power from light from the lighting infrastructure. The receivers can also include a transmitter for retransmitting the receive position data and an ID code to a central processing center.

FIELD OF THE INVENTION

The present invention relates to terrestrial navigation and positioningsystems. More particularly, the present invention relates to an extendedcapability global positioning system (GPS) capable of resolving positionwithout access to line-of-sight satellite, pseudolite or cellulartelephone signaling.

BACKGROUND OF THE INVENTION

Global positioning system (GPS) navigational systems are often used bymilitary and civilian naval, ground, and airborne vehicles fornavigation. GPS receiver units receive positioning signals from aconstellation of 24 Navistar satellites deployed in 12-hour orbits aboutearth and dispersed in six orbital planes at an altitude of 2,200kilometers. The satellites continuously emit electronic GPS signals (ortelemetry) for reception by ground, airborne, or naval receiver units.By receiving GPS signals from four or more satellites, a properlyconfigured receiver unit can accurately determine its position in threedimensions (e.g., longitude, latitude, and altitude).

GPS navigational systems have tremendous benefits over other positioningsystems in that these systems do not rely upon visual, magnetic, orother points of reference. However, conventional GPS navigationalsystems can experience blackout areas or regions when line-of-site isbroken with the satellites. Some GPS navigational systems or othernavigational systems in use today are nonfunctional in many areas due tosignal blockage. For example, personal navigational systems oftenexperience loss of signal when they are operated indoors, in dense urbanenvironments (e.g., urban canyon), next to large buildings, underground,or in other blackout areas. Additionally recent attempts at augmentingGPS for this coverage deficiency has had mixed success and not resultedin a low cost worldwide solution. Additionally, approaches for resolvinglocation through the novel use of cellular phone infrastructures doesnot currently provide reliable or sufficient discrimination forresolving occupied floor level within a multi-story concrete and metalbuilding. Similarly, the approach of using GPS pseudolites does notsolve all coverage problems due to inherent signal distortion,reflection and attenuation again brought on by the use of concrete andmetal in many buildings. Furthermore the payback incentives are notclear, resulting in questions of who should pay for the newinfrastructure, undermining solution standardization and widespreadsystem implementation and coverage.

Thus, there is a need for a navigational system which not only is lesssusceptible to blackout areas, but is inherently low cost and havingcharacteristics attracting widespread or worldwide infrastructuresupport. Further still, there is a need for a low-cost personal locationsystem which can be utilized to determine position indoors. Even furtherstill, there is a need to extend satellite navigation systems so theycan be used indoors and in urban canyon environments.

SUMMARY OF THE INVENTION

The present invention relates to a transmitter disposed at a positionfor use in a positioning system. The transmitter includes a wirelesspower circuit and a control circuit. The control circuit is coupled tothe wireless power circuit. The wireless power circuit provides electricpower to the control circuit. The control circuit transmits a limitedrange positioning signal indicative of the position.

The present invention further relates to a receiver unit for use with aplurality of transmitters. Each transmitter emits a limited rangereceive position signal indicative of a position of the transmitter. Thereceiver unit includes a receiver circuit for receiving the limitedrange receive position signal, a transmitter circuit, and a storagebuffer. The storage buffer is coupled to the transmitter circuit. Thestorage buffer stores an identification code. The transmitter transmitsa transmit position signal indicative of the limited range receiveposition signal and the identification code.

The present invention still further relates to a positioning systemreceiver including a satellite receiver circuit and an auxiliaryreceiver circuit. The satellite receiver circuit receives satellitesignals and determines a first position in response to the satellitesignals. The auxiliary receiver circuit receives limited range signalsand determines a second position in response to the limited rangesignals. The limited range signals include a positioning code. Theauxiliary receiver circuit utilizes the position code to determine thesecond position.

According to one exemplary aspect of the present invention, a globalterrestrial based system includes uniquely programmed miniaturetransmitters, each with limited broadcast range and massively deployedin fixed locations for use within a positioning system. Thesetransmitters each include a wireless power circuit, control circuit,programmable memory buffer, radio frequency (RF) oscillator-amplifier,and antenna system. In its most ideal form, the power circuit isdesigned to obtain its operating energy parasitically and wirelesslyfrom a pre-existing distributed utility infrastructure (e.g. street andbuilding lighting, telephone, a.c. power etc.), converting and providingelectric power to the control, memory, and RF circuits. The controlcircuit is designed to repeatedly deliver (about once a second) a uniquepre-programmed code from the memory buffer to the RE amplifier. Theresultant modulated RF carrier signal broadcast is regulated to belimited in propagation range. The broadcast range and on-the-air dutycycle may be selected at the time of installation for optimumperformance with intended user navigation habits and available power.E.g., Variables such as typical user travel speed and typical navigatordistance from said transmitter are factors that would influence desiredeffective radiated broadcast power and data broadcast periodicityrespectively. Broadcast power, broadcast frequency, antenna design, andpower supply are selected and installed accordingly, ensuring maximallikelihood of navigator signal reception. The pre-programmed positioncodes stored within each transmitter memory unit are uniquely prescribedat the time of installation, or in advance, based on survey or othermathematically derived means. Passing navigator signal data receptionyields discrete information points indicative of last known general userposition. The transmitters may be fielded at a variety of adjacentdistances and power levels providing broadcast coverage ranging from acontinuous two or three dimensional fine grid, to a one-dimensionaldiscontinuous corridor arrangement.

In accordance with another exemplary aspect of the present invention, areceiver unit is for use with plurality of asynchronous griddedtransmitter units. The receiver unit is capable of detecting low-powerdigital broadcast transmissions from any one of the independentlyfielded transmitter units, provided that the receiver distance is withinlocal broadcasting range (a few feet to several hundred feet). Areceiver discrimination circuit accepts the strongest navigation signalexceeding a preset detected energy level. Additionally the receiverincludes a data buffer, display, and/or an optional embedded repeatercircuit for the purpose of further relaying of received position dataalong with a user receiver identification code. Other parties equippedwith a appropriately tuned receiver may independently monitornavigator's position.

The present invention still further relates to a unified receiverpositioning system that incorporates a satellite GPS receiver with theaforementioned receiver invention. In response to the valid detection ofa GPS satellite signal (first receiver), the unified system reports tothe user interface a GPS position location indication. In response tothe valid detection of a local fixed point broadcast signal from theterrestrially based transmitter grid (second receiver), the unifiedsystem may additionally report to the user, secondary receiver positiondetection, or in the event of the extended loss of reliable GPS data,secondary receiver data becomes primary. Upon regaining reliable GPSsatellite signaling, the unified receiver control reselects the GPS dataas primary. Receiver "hand-off" time constants are made programmable forparticular acceptable use.

In yet another exemplary aspect of the present invention, the limitedrange low-power fixed point broadcast transmitter grid is provided inconcert with a pre-existing lighting infrastructure (e.g. buildingcorridor and/or street lights). The miniature transmitters parasiticallyand wirelessly derive power from the lighting infrastructure via a smallsolar cell. The transmitters are preferably affixed to the lightingfixtures, may be integrated with lighting hardware, or may betemporarily associated with or permanently part of a bulb design.Additionally the transmitters can utilize existing or additionalmounting or reflecting hardware to focus, attenuate, or otherwisecontrol RF propagation of the position signal. Pre-existing lightinginfrastructure is generally designed and appropriately concentrated foruse benefiting human navigation. It is noted that the relationship ofexisting navigational lighting infrastructure in urban areas to beintentionally and highly correlated with building and transportationinfrastructure density due to solar light broadcast attenuation andevening use. In a very related manner, it is these infrastructures thatare also the primary cause for reduced performance and availability ofGPS signal broadcasts to these same areas. An exemplary aspect of thepresent invention is the recognition of these similar problems, bothrelating to navigation enhancement, and addressed by uniting the use ofexisting world-wide point source lighting infrastructures with pointsource RF navigation related broadcasting. Invention application henceresults in the simultaneous navigation utility extension of pre-existingworldwide urban lighting infrastructure and GPS satellite basedpositioning systems. Furthermore, a unique set of conditions essentialfor the feasibility of low cost global implementation has been addressedand realized by the invention. The power circuit design using theprinciple of wireless and parasitic energy derivation from saidassociated lighting infrastructure, significantly reduces requiredinstallation labor time and skill level. It is additionally noted thatthe invention and its power circuits uniquely meet a balance ofproviding adequate power for a required useful point broadcast whilehaving minimal obtrusive effect on intended fixture lightingperformance.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will hereafter be described with reference to theaccompanying drawings, wherein like numerals denote like elements, and:

FIG. 1 is a schematic block diagram of an urban canyon environmentemploying transmitters in accordance with an exemplary embodiment of thepresent invention;

FIGS. 2A and 2B show the employment of the transmitters illustrated inFIG. 1 upon lighting hardware in accordance with another exemplaryembodiment of the present invention;

FIG. 3 is an electrical schematic drawing of the transmitter illustratedin FIG. 1;

FIG. 4 is a schematic block diagram of a receiver for use with thesystem illustrated in FIG. 1 in accordance with another exemplaryembodiment of the present invention;

FIG. 5 is a more detailed schematic block diagram of the receiverillustrated in FIG. 4; and

FIG. 6 is a schematic block diagram showing the transmitter illustratedin FIG. 2B being programmed in accordance with still another exemplaryembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, an urban canyon environment 10 includes abuilding 12 situated on a street corner 14. Street corner 14 includes astop light 16 and street lights 18. Building 12 is comprised of a numberof floors 20 which include lights 22.

Urban canyon environment 10 relates to any area or location wheresatellite signals, such as, signals from global positioning system (GPS)satellites, such as, satellite 23 are detrimentally attenuated, blocked,or distorted by geological or man-made structures. For example, building12 may prevent the satellite signals from reaching street corner 14 orGPS black out zone 15. Additionally, building 12 may prevent thesatellite signals from reaching areas on floors 20 within building 12.Environment 10 can be any location where there is not a clearline-of-sight to positioning satellites or where the satellite signalscannot be accurately received.

In accordance with an exemplary embodiment, the global positioningsystem (GPS) is augmented with transmitters 30 throughout environment10. Transmitters 30 are linked to the lighting infrastructure (e.g.,lights 22 and street lights 18) within building 12 and on street corner14. For example, transmitters 30 can be attached or integrated with thelight bulbs or lighting hardware associated with lights 22 or streetlights 18. Similarly, traffic light 16 may include a transmitter 30. Byutilizing the lighting infrastructure associated with environment 10,transmitters 30 can provide positioning information to environment 10 atsubstantially less cost since the hardware associated with street lights18 and lights 22 is already present and in significant concentration foruse relating to human navigation. Placement and use scenarios caninclude public or private highways, roads, streets, parking facilities,bridges, overpasses and tunnels etc. Additionally building stairways,hallways, offices, rooms, auditoriums, corridors, and basements etc. canbe complemented with devices.

Transmitters 30 are preferably very low power transmitters whichbroadcast unique position codes as a low-level limited range positionsignal. The position signal includes position information modulated on acarrier wave. Preferably, the useful range of transmitters 30 isselected to be several feet to a few hundred feet. For example, therange of transmitter 30 can be three feet, ten feet, or other distancenecessary to accurately approximate the position of a receiver (notshown in FIG. 1) in environment 10.

Transmitters 30 are preferably programmed, as discussed below withreference to FIG. 6, to broadcast a particular position code or programposition signal with each location. Transmitters 30 can be programmedvia a hardwire, optical, or RF programming technique. The positionsignal can include three-dimensional position information, such as,longitude, latitude, and altitude or street address, floor number,building name, room number, or other location information. Unlikesatellite signals, the broadcast is directly encoded with uniqueposition signal. Thus, position information does not need to be derivedfrom a comparison of synchronized signals from multiple sources, as in aGPS navigational system.

Transmitters 30 also advantageously utilize the lighting infrastructureto derive power. For example, transmitters 30 can utilize energyassociated with lights 18 and 22 via solar cells or panels. inductivea.c. power coupling or other techniques, for deriving energy from thelighting infrastructure. Preferably, transmitters 30 do not require adirect electrical connection between the lighting infrastructure toderive power (e.g., wireless derivation of power). Transmitters 30 mayinclude recharging capacitor and battery circuits to store power whenlights 22 or street lights 18 are off or during twilight periods.Further, transmitters 30 disposed on street lights 18 and traffic light16 can derive power from the solar broadcast of sun 26 during daylighthours.

With reference to FIGS. 2A and 2B, transmitters 30 can be disposed onlight shield 32 or light shield 34. Light shield 32 is associated with astreet light 18 or an incandescent light bulb 35, and light shield 34 isassociated with fluorescent light tubes 37. Shields 32 and 34 as well asdiffusers 33 and 39 provide a convenient space for attachingtransmitters 30 and operate to focus, or to act as a radio frequency(RF) attenuator or shield that inhibits or limits the broadcast of theposition signal to a particular zone or area. For example, if shield 34is utilized on lights 22, transmitter 30 is less likely able to transmitits RF or position signal above to another floor 20 because shield 34blocks the transmission of the position signal. Preferably, the RFfrequency of the position signal from transmitter 30 is chosen to bebeneficial with the size and shape of shields 32 and 34.

Transmitters 30 can be attached to reflectors, diffusers, transformers,ballasts, bulbs, sockets, connectors, bases, filliments, lenses orshields (32 and 34), by a magnet, a threaded housing, an adhesive 31,mechanical hardware (e.g., brackets and screws), Velcro™, or otherattachment devices. Alternatively, transmitters 30 can be integratedwithin a light bulb 35 or light tubes 37. In an other alternative,transmitters 30 can be provided in toroidal or doughnut-shaped packages41 and 43 which surround bulb 35 or tube 37, respectively. Light bulbs35 and light tubes 37 can be halogen bulbs, sodium vapor bulbs,fluorescent bulbs, or incandescent bulbs.

With reference to FIG. 3, an electrical schematic drawing of transmitter30 includes a battery or capacitor or battery/capacitor 42, a diode 44,a solar cell 46, an infrared interface 48, a control circuit 50, acrystal 52, a field effect transistor (FET) 54, a capacitor 56, a loopand/or straight antenna 60, and a capacitor 62. Diode 44 can be removedfrom transmitter 30 if solar cell 46 is utilized. The operation oftransmitter 30 is discussed below with reference to FIG. 3.

Solar cell 46 is a wireless power circuit which receives light from thelighting infrastructure associated with building 12 and street corner 14(FIG. 1) and generates power which is stored in battery/capacitor 42.Preferably, solar cell 46 provides three volts at node 66 in response tolight from lights 16, 18, 22, or sun 26. Alternatively, cell 46 can bean inductive transformer or other device for parasitically derivingpower without direct electrical contact. Control circuit 50 receives thepower signal at node 66 and provides a gate signal representative ofposition data stored in circuit 50 to a gate 10 of FET 54.

The gate signal is mixed with a synthesized signal from the oscillatorwhich is comprised of crystal 52, antenna 60, capacitor 62, FET 54 andcapacitor 56. Preferably, the oscillator source is a carrier signalchosen for optimal broadcast propagation and attenuative characteristicswithin buildings. Additionally, the oscillator frequency is chosen forlow circuit energy dissipation, small antenna profile and also withregard to available frequency band (e.g., 49.00 or 800 Mhz ISM frequencybands), with a small loop and/or straight wire antenna). FET 54 providesa resultant modulated carrier signal through antenna 60 as the positionsignal. The position signal includes the gate signal which representsthe position data associated with the location of transmitter 30. Thus,the position signal includes the position data encoded onto the carriersignal through amplitude modulation (AM). Alternatively, othermodulation (narrow band and wide band) or encoding techniques can beutilized to transmit a position signal representative of the positiondata.

Control circuit 50 can include digital logic, microcontroller, or otherdevice for generating the gate signal provide to gate 70. Controlcircuit 50 is preferably a custom-designed integrated circuit forperiodically providing the gate signal representative of the positiondata to gate 70. The components of transmitter 30 (an be mounted to oron solar cell 46. Control circuit 50 includes a buffer or memorycircuit, a sensor interface, and a pulse generator for providing thegate signal to gate 70. Alternatively, the position signal can bevisible or infrared light, audio, laser, electromagnetic, or other typeof signal. The components of ET transmitter 30 control circuit 50.

Additionally, the memory within control circuit 50 can be programmed byproviding signals through infrared interface 48 or in combination orexclusively with panel 46 by modulating the power bias input.Alternatively, infrared interface 48 can be replaced by an RF interface,a hard wire interface, or other programmable system. Preferably, theposition data is field programmable into the memory in control circuit50 as discussed with reference to FIG. 6. The memory in control circuit50 is non-volatile.

Antenna 60 is preferably a loop, wire, or hybrid combinationbroadcasting antenna of relatively small size, optimized for broadcastpropagation control. The position signal, provided through antenna 60,preferably has a very low amplitude level, ranging from sub-milliwatt to10's of milliwatts depending on application requirements.

With reference to FIG. 4 and 5, a positioning receiver 100 for use inenvironment 10 includes a display 102, an antenna 104, and auxiliaryreceiver 106, and a GPS receiver 108. Antenna 104 can be one, two, ormultiple antennas. Receiver 100 also includes a control circuit 120(FIG. 5), a buffer 122, a programmable interface 124, and a transmitcircuit 126. Receiver 100 can be a hand-held receiver or otherwiseattached to a person. Alternatively, receiver 100 can be attached to avehicle. Receiver 100 can also be part of a two-way transceiver, acellular telephone or a GPS receiving system.

Control circuit 120 is coupled to display 102 to provide information todisplay 102, providing visual indicia of the position of receiver 100.Control circuit 120 receives position data from auxiliary receiver 106and from GPS receiver 108 to provide the display signal to display 102.Preferably, control circuit 120 indicates on display 102 from whichreceiver 106 or 108 the position data is received.

Auxiliary receiver 106 receives position signals from transmitters 30 inenvironment 10 (FIG. 1) and provides the position data to controlcircuit 120. Similarly, GPS receiver 108 receives GPS satellite signalsfrom satellites on antenna 104 and provides the position data to controlcircuit 120. Generally, control circuit 120 chooses the position datafrom receiver 108 because it is often available.

As a user enters urban canyon environment 10, control circuit 120determines that satellite signals are not being received by GPS receiver108 and determines if position signals are being received by auxiliaryreceiver 106. If position signals of high enough levels are beingreceived by auxiliary receiver 106, and the satellite signals are notbeing accurately received by receiver 108, control circuit 120 selectsthe position data from auxiliary receiver 106. Alternatively, controlcircuit 120 can wait a predetermined amount of time from when GPSreceiver 108 no longer receives satellite signals before analyzing theposition signals received by auxiliary receiver 106. In this way,receiver 100 can be utilized in urban canyon environment 10 and also innon-urban canyon environments. Therefore, receive 100 can provideaccurate positioning wherever transmitters 30 are provided or satellitesignals can be received.

Receiver 100 can also be utilized as a personnel locator. For example,receiver 100 can be worn or carried by a fire-fighter in a fire-fightingsituation. If building 12 were on fire, receiver 100 could be utilizedby a fire-fighter not only to determine a position in building 12, butalso to relay that position to a fire truck, a fire station, or othercontrol centers. In such an embodiment, control circuit 120 wouldprovide the position data from receiver 106 or receiver 108 andidentification data from buffer 122 to transmit circuit 126. Transmitcircuit 126 would transmit the position data and the identification datain a transmit signal via antenna 104. Preferably, for fire-fightingsituations, transmitters 30 and transmit circuit 126 emit RF signalswhich are not dramatically inhibited by smoky environments.

Buffer 122 stores the identification data or ID code. Buffer 122 can bea dip switch, PROM, S-RAM, FLASH PROM or other programmable device forlong memory. Buffer 122 can be programmed via an interface 124 similarto interface 48 with reference to FIG. 3. Alternatively, programmableinterface 124 can be switches, an RF interface, or other device forsetting an ID code in buffer 122.

Transmit circuit 126 can be similar to transmitter 30. However, transmitcircuit 126 preferably has significantly more power to transmit theposition and identification data (transmit signals) farther distances.Control circuit 120 can be configured so transmit circuit 126rebroadcasts the position signal each time new position data is detectedby receiver 106 or by receiver 108. The rebroadcasted signal can bereceived by a fire truck, law officer or other command center authorizedto receive such information. Additional system uses include vehiclelocation, personnel location, cellular telephone transmit powerrequirement estimations, emergency 911 locating, personnel or assettracking. In addition to navigation uses, said point broadcasttransmitters may be equipped and/or designed to accept and store otherinformation relating to position environment, including sensory data forassociative broadcast with position code. Uses may include hazardousmaterial awareness or building heating, ventilating, andair-conditioning (HVAC) sensory feedback system uses. The latter usewould include a sensory interface and data buffer as part of theminiature transmitter design. The position location broadcast would theninclude limited environmental history data for optimal heating andcooling regulation. Regular cleaning or maintenance crews couldpassively collect sensory data history via a personal receiver for laterdownloading and use at a central building HVAC control center. Thesesafety, communication and energy use aspects address worldwide concernsproviding natural incentive for widespread system implementation.

With reference to FIG. 6, before or after transmitter 30 is installed onshield 34, a hand-held unit 76 coupled to a position location codegenerator 78 can provide prescribed position data to control circuit 50within transmitter 30 (FIG. 3). Shield 34 is coupled to a ballasttransformer 79 which can inductively provide power to transmitter 30.Position location code generator 78 can include a calibrated inertialpositioning system or other device for providing the position codethrough wand 76. Generator 78 can also be coupled to a bar code printer81 for providing a bar code sticker 83 which can he affixed to shield34. Sticker 83 includes position information which can be utilized forfuture servicing of transmitter 30. Wand 76 can be configured to scanand read sticker 83. Alternatively, generator 78 can simply include auser interface for manually inputting the position data based upon maps,charts, or other devices. Wand 76 transmits the position data tointerface 48 within transmitter 30 (FIG. 3).

It is understood that, while the detailed drawings, specific examples,and particular components values given describe preferred embodiments ofthe present invention, they serve the purpose of illustration only. Theapparatus of the invention is not limited to the precise details,distances, voltages, frequencies, and conditions disclosed. For example,although particular wireless power systems are disclosed, other types ofpower systems car be utilized. Further, single lines in the various canrepresent multiple conductors. Various changes can be made to thedetails disclosed without departing from the scope of the spirit of theinvention which is defined by the following claims.

I claim:
 1. A receiver unit for use with a plurality of transmitters,each transmitter emitting a limited range receive position signalindicative of a position of the transmitter, the receiver unitcomprising:a receiver circuit for receiving the limited range receiveposition signal; a transmitter circuit; and a storage buffer coupled tothe transmitter circuit, the storage buffer storing an identificationcode, the transmitter circuit transmitting a transmit position signalindicative of the limited range receive position signal and theidentification code.
 2. The receiver unit of claim 1, wherein thestorage buffer is an EPROM.
 3. The receiver unit of claim 1, wherein thestorage buffer is a dip switch.
 4. The receiver unit of claim 1, whereinthe receiver unit is field-programmable to provide the identificationcode.
 5. A unified positioning receiver system, comprising:a satellitereceiver circuit for receiving satellite signals and determining a firstposition in response to the satellite signals; and an auxiliary receivercircuit for receiving limited range signals and for determining a secondposition in response to the limited range signals, the limited rangesignals including a position code, the auxiliary receiver circuitutilizing the position code to determine the second position.
 6. Thepositioning receiver system of claim 5, wherein the satellite signalsare provided in a first frequency band and the limited range signals areprovided in a second frequency range.
 7. The positioning receiver systemof claim 5, wherein the limited range signals have a range of less than250 feet.
 8. The positioning receiver system of claim 5, wherein thelimited range signals have a range of less than 15 feet.
 9. Thepositioning receiver system of claim 5, further comprising:an antennasystem coupled to the satellite receiver circuit and to the auxiliaryreceiver circuit.
 10. The positioning receiver system of claim 5,further comprising:a transmitter circuit; and a storage buffer coupledto the transmitter circuit, the storage buffer storing an identificationcode, the transmitter circuit transmitting a transmit position signalindicative of the identification code and the first position or thesecond position.
 11. The positioning receiver system of claim 10 furthercomprising:an interface coupled to the storage buffer, the interfaceproviding the identification code to the storage buffer.
 12. Thepositioning receiver system of claim 5 further comprising:a display forproviding an indication of the first position or the second position.13. The positioning receiver system of claim 5, wherein the limitedrange signals are RF signals.
 14. The positioning receiver system ofclaim 5, wherein the limited range signals are infrared signals.